1. Field
The present embodiments generally relate to systems and methods for heating cryogenic fluids. More particularly, embodiments relate to systems and methods for heating liquefied natural gas (“LNG”) using an environmentally friendly three shell heater design.
2. Description of the Related Art
Since liquefied natural gas (“LNG”) occupies approximately 600 times less volume than an equivalent weight of gasified natural gas, the liquefied form of natural gas is the preferred method for economical, large scale, intercontinental, shipment of LNG. Most modern LNG tankers range in size from 50,000 m3 to in excess of 200,000 m3. A 120,000 m3 LNG tanker is capable of transporting the equivalent of approximately 74 million standard cubic meters (2.6 billion standard cubic feet) of natural gas, or the per capita usage of approximately 35,000 people. However, the handling of such large volumes of gas requires significant fixed assets be dedicated to both the liquification of the natural gas at the port of departure and regasification of the LNG at the port of arrival.
Upon arrival at a destination port, the LNG is vaporized prior to introduction to one or more natural gas distribution networks. With a heat of vaporization of approximately 550 kJ/kg, and a bulk density of approximately 445 kg/m3, the vaporization of relatively small quantities of LNG requires significant heat. For example, complete vaporization of 120,000 m3 of LNG will require approximately 2.9×1010 kJ (2.7×1010 BTU). In many circumstances, hot water or steam is used to provide the heat required to vaporize the LNG. Unfortunately, systems based upon the use of water or steam as a heating media are prone to freezing due to the low boiling point of the natural gas (−162° C.). Freezing impairs the efficiency of the vaporization process, requiring more heat transfer surface area than if the icing could be avoided.
The evaporators presently used are mainly of the open rack type, intermediate fluid type and submerged combustion type. Open rack type evaporators use sea water as a heat source for vaporizing the LNG. These evaporators use once-through seawater flow on the outside of a heat exchanger as the source of heat. Untreated sea water, however, often contains substantial quantities of suspended solids which can foul the evaporator, thereby reducing the heat transfer efficiency and increasing the time required to vaporize the LNG. In addition to the potential fouling of the evaporator, regasification using sea water can cause thermal pollution in the surrounding estuarine waters. Thus, the use of open rack type vaporizers is often not the system of choice because of environmental reasons. Regasification using estuarine waters as a heating medium is discussed in U.S. Pat. Nos. 6,089,022, 6,164,247, and 6,598,408.
Intermediate fluid type evaporators use propane, halogenated hydrocarbons or similar refrigerants having a low freezing point to supply heat to LNG instead of using direct heating with water or steam. The refrigerant is usually heated with hot water or steam to provide both the sensible heat and heat of vaporization of the refrigerant for heating the LNG. Intermediate fluid type evaporators are typically less expensive to build than those of the open rack-type but intermediate fluid type evaporators consume a portion of the LNG as fuel to heat the refrigerant. A typical intermediate fluid type evaporator can consume between 1.5% and 3% of the total LNG vaporized as fuel.
Submerged combustion type evaporators, i.e. submerged combustion vaporizers (“SCV”) typically contain a heated combustion chamber containing an LNG fired burner immersed in a liquid bath. SCVs can provide an efficient alternative to other types of fired heaters; however, SCVs also consume a portion of the vaporized LNG product to provide the heat for vaporization. While avoiding potential freezing issues encountered when using water as a heating medium, local, state and federal air permits may be required to operate an SCV, additionally NOx emissions from an SCV may require selective catalytic reaction (SCR) to achieve permit compliance. U.S. Pat. No. 7,168,395 discusses the use of a submerged combustion LNG vaporizer using a gas fired submerged heater.
There is a need, therefore, for an improved system and method for vaporizing LNG and other cryogenic fluids without the risk of freeze-up, with minimal environmental impact and with minimal permitting requirements.